We picked up some GF pasta in a small supermarket in Rethymno.
Alana Restaurant in Rethymno shows GF on the menu. It's a lovely restaurant in the old town area near to the Fortezzi (old Fortress) and some lovely tourist shops selling traditional products.
We saw no GF at Heraklion airport.
Also the M&S sold on board the British Airways flight had no gluten free food other than a few crisps, a few olives, some fruit and nuts and a GF chewy bar. (We had a severe nut allergy customer on our flight so we couldn't eat anything with nuts in it anyway - so we could only have bought crisps and olives). Fortunately we had taken sandwiches with us. (NB. Take a packed lunch without any nuts).
Another point away from gluten free food - there was no entertainment at all on a 4 hour flight to a holiday destination and the drinks trolley (to purchase drinks) only came round once. A poor show from BA!